Asymmetrical balance beam



ASYMMETRICAL BALANCE BEAM Filed July 18, 1961 United States Patent flicc 3,029,883 Patented Apr. 17, 1962 3,029,883 ASYMMETRICAL BALANCE BEAM Bojan Kriznic, Kusnacht, Zurich, Switzerland, assignor to Erhard Mettler, Zoliikon, Zurich, Switzerland Filed July 18, 1961, Ser. No. 124,874 Claims priority, application Switzerland Nov. 7, 1960 7 Claims. (Cl. 177-246) This invention relates to an asymmetrical balance beam for highly sensitive analytical or microbalances. Such balance beams consist of an edge arranged plate-shaped member forming the balance beam arms, a centre knifeedge fastened thereto, an end knife-edge on one of its arms, and a counterweight on the other of its arms. The two knife-edges extend perpendicular to the plane of the plate-shaped member and preferably project from rigid knife-edge supports, which are fastened to the plateshaped member. The present invention aims to produce a balance beam of the aforementionedasymmetrical construction, the sensitivity of which, within certain limits, is independent of the ambient temperature.

The invention accordingly provides an asymmetrical balance beam of the type stated having a slot penetrating from the edge of said plate-shaped member in a direction transversely to the longitudinal direction of the balance beam, and a connecting member spanning said slot and holding together the two sections of the plate-shaped member on either side of said slot, said connecting memher being made of a material having a linear coeiiicient of expansion different from that of the material of which said plate shaped member of the balance beam is made.

Preferably, said connecting member is of H-shaped crosssection, the web of which'fits into said slot, while the two parallel branches of the connecting member overlap externally the two sections of the balance beam oneither connecting member must be made of a material which has a lower linear coefiicient of expansion than the material of which said plate-shaped member is made.

These and other objects and the advantages of the invention will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing in which:

FIG. 1 shows a side view of an asymmetrical balance beam, and 1 FIG. 2 shows a top view of the balance beam shown in FIG. 1.

The balance beam has a plate-shaped member 3 of equal thickness throughout, which is provided intermediate its ends with a rigid knife-edge support 5 projecting from the bottom edge 4 of member 3. A downwardly extending central knife-edge 6 is fastened to the support 5, the knife-edge 6 extending in a direction transversely to the longitudinal direction of the balance beam. To the shorter arm 7 of the balance beam is fastened another, somewhat shorter knife-edge support 8, can-ying an upwardly extending outer knife-edge 9 which is parallel to the knife-edge 6. The longer balance beam arm 10 has at its end a bolt 11, onto the threaded shank 12 of which is screwed a counterweight 13 comprising in the conventional manner a plurality of round discs. On the longer arm 10 beyond the bolt 11 there is fitted a socket 14 to hold a graduated or numbered dial plate 15 for reading the inclination of the balance beam when weighing, and thereby the last decimal figures. of the object to be weighed. I

The plate-shaped part 3 of the balance beam is made, as usual, of aluminium alloy, so that the weight of the whole beam is small. In the longer beam arm 10 and adjacent the centre knife-edge 6 a slot 17 is formed extending downwards from the upper edge 16 of the plate- "of which the plate-shaped member 3 is made.

shaped member. As is shown in FIG. 1, the slot 17 extends to a height greater than half the height of the balance beam at the place of the slot 17. Into this slot 17 is inserted a connecting member 18 which, as shown in FIG. 2, is substantially H-shaped in cross-section. The Web of this connecting member 18 fits into the slot 17 while the two parallel branches of the connecting member 18 span the slot 17, so that the two sections of the balance beam on either side of the slot 17 are overlapped on the outside by the atom-mentioned branches of the member 17, as shown in FIG. 2. The two branches of the member 17 are attached to the adjacent parts of the balance beam, which they span, by cementing with synthetic resin and pinning. It is also possible to fasten the connecting member 18 to the embraced parts of the beam by other means, provided that these give an intimate attachment which is able to sustain considerable shearing forces. As FIG. 1 shows, the height of the connecting member 18 is less than half the weight of the slot 17, and the connecting member 18 is arrangedflush with the upper edge 16 of the plate-shaped member 3. The connecting member 18 is made of a material having a lower linear coefiicient of expansion than the aluminiumalloy A conhecting member 18 of anti-magnetic stainless steel has been proved to be suitable; the linear expansion coefficient of such a steel is only about half that of the aluminium alloy used for balance beams. The connecting member 18 is further fastened to the two parts of the balance beam on either side of the slot 17 in such a way that, at the normal temperature of for instance 20 C., no mechanical stresses arise within the balance beam.

If the temperature of the balance beam is increased, the area 19 of the balance beam between the lower end of the slot 17 and the lower edge of the plate-shaped member 3 will expand more markedly than the connecting member 18. The latter exercises a tensile action which endeavors to raise the free end of the longer balance beam arm 10. At a higher temperature however, the coefficient of elasticity also increases. The counterweight 13 causes fiexural loading of the longer arm 10,

the free end of which shows a tendency to move downwards. Thus the dimensions of the slot 17 and of the connecting member 18 can be adjusted in such a way that these two aforementioned phenomena arising when the temperature is raised compensate for each others effects to the extent that the sensitivity of the entire balance beam remains constant within broad limits. When the temperature drops, analogous reactions are produced in the reverse sense. In this way it is possible to achieve virtually constant sensitivity of the balance beam within a temperature range of up to 60 C., i.e. for instance between 10 and +50 C., which includes all working temperatures which can reasonably be considered for an analytical or micro balance.

In the embodiment in FIGS. 1 and 2 the slot 17 is directed downwards from the upper edge of the member 3. The slot 17 could also, however, extend upwards from the lower edge 4 of the plate-shaped member 3. In that case, though, the connecting member 18 would have to be made of a material showing a greater coefiicient of expansion than the material making up the plate-shaped member 3 of the balance beam. Supposing that an aluminium alloy is used for the member 3, a zinc connecting member 18 would be appropriate in view of the required coeflicientof expansion. In view of its other mechanical properties, however, a connecting member 18 made of zinc would hardly appear to be suitable. For an aluminium beam, therefore, virtually the only possible construction is one in which the slot extends downwardly from the upper edge 16, as in FIGS. 1 and 2. In that case a fair number of materials having both the required mechanical properties and a consider-ably lower coetficient or expansion than aluminium are available for the connecting member 18. Stainless and anti-magnetic steel and also titanium have shown to be the best materials for theconnecting member 18.

Ihe slot 17 could furthermore be made at a point in the balance beam other than thatshown in FIGS. 1 and 2. For instance, -practically the'same eltect could be produced by inserting the connecting member .18 into the balance beam flush with the upper part of the plateshaped member 3 in an area above the centre knife-edge 6. But the connecting member '18 can also be further removed from the centre knife-edge 6 and placed approximately in the middle oi the longer beam arm 16. It would also be possible to, insert the connecting member v18 in the shorter beam arm 7. These and similar variants have the disadvantage, however, that the results sought are not achieved without stronger mechanical tenisiletand compressing forces arising, which intact is undesirable. The most suitable place to insert the connect-V 'ing member 18 is an area near to and above the central knife-edge 6 and'still in the longer balance beam arm 10, as is already shown in FIGS. 1 and 2. Finally, the balance beam may also be provided with several slots in various places and connecting members inserted therein. These and similar modifications of the balance beam which may occur to those familiar with the art fall within the spirit and .scope of the invention as set forth in the following claims. I-claim: 1. An asymmetrical balance beam for analytical or .micro balances having an edge arranged plateshaped member forming the balance beam arms, a centre knifeedge secured to said member, an end knife-edge on one of its arms 'and a counterweight on the other of its arms,

said two gknife-edges extending perpendicular to the plane of said plate-shaped member, said plate-shaped member having a slot penetrating from the edge of the plateshaped member in a direction transversely to the longitudinal direction of the balance beam, anda connecting member spanning said slot and holding together the two sections on eitherside of said slot, said connecting member being made of a material having a linear coefficient of expansion different from that of the material'of which ,said plate-shaped member of the balance beam is made.

2. The invention as recited in claim 1, wherein said connecting membervis of H-shaped cross-section, the web 4 of which fits into said slot, while the two parallel branches of said connecting member overlap externallythe two sections of the balance beam on either side of said slot.

3. The invention as recited in claim 1, wherein said slot and said connecting member are situated in an area near to the centre knife-edge on the longer balance beam 7 arm, said longer balance beam-arm having the counterweight. 1

4. An asymmetrical balance beam for analytical or micro balances having an edge arranged vplate-shaped member forming the balance beam arms, a centre'knifecdge secured to said member, an end knife-edge on one of its arms and a counterweighton the other of its arms, said two knife-edges extending perpendicular to the plane of said plate-shaped member, said plate-shaped member having a slot which is directed downwardlyfrom the'upper edge of said plate-shaped membenand a connect-V ing member spanning said slot and holding together the member of equal thickness forming the balance beamarms, a centre knife-edge secured to said member, a end knife-edge on one of its arms and a counterweight on member of its arms, said two ,knife-edge-stextending perpendicular to the plane of saidplate-shaped member, :said plate-shaped member having a slot penetrating rtrom the edge of the plate-shaped member over a height at least equal to half the height of the balance beam in the region oi'said slot, and a connecting member spanning :said slot and holding together the two sections of said plate-shaped member on either side of said slot, said?- connecting member being made of a ,rnaterial having a linear coefficient of expansion different from that of the material of which said plate-shaped member is made.

7. The invention as recited in claim 6, wherein the height of said connecting member is .less' than half the height of said slot, and also said connectingmember is arranged on one side flush with the edge from which said slot penetrates.

No references cited. 

